Quick reference
Scan this table first. Details for each row are below.
| What you're taking | Level | Short version |
|---|---|---|
| MAOIs | Contraindicated | Risk of hypertensive crisis — 14-day gap either side |
| Alcohol and CNS depressants | Caution | May add to dizziness and cognitive effects |
| Oral contraceptives | Generally okay | Spotting may increase — do not stop the pill |
| Non-potassium-sparing diuretics | Caution | Potassium measured before and during treatment |
| Phenytoin or carbamazepine | Caution | Lower topiramate levels by 40–48% |
| Valproic acid | Caution | Hyperammonaemia and hypothermia reported with topiramate |
| Other carbonic anhydrase inhibitors | Avoid or discuss | Worse metabolic acidosis and more kidney stone risk |
| Amitriptyline | Caution | Levels can rise sharply in some people |
| Pioglitazone | Caution | Exposure falls — consider more glucose monitoring |
MAOIs
The one combination the Qsymia interactions table rules out completely.
Monoamine oxidase inhibitors
Avoid or discussMAOIs are an older type of antidepressant. Taken with phentermine they raise the risk of a hypertensive crisis — a sudden, severe rise in blood pressure. The label says Qsymia must not be taken during MAOI treatment.
The label says taking phentermine at the same time as a monoamine oxidase inhibitor (MAOI) increases the risk of hypertensive crisis. So Qsymia is contraindicated — the label says not to take it — during MAOI treatment, and within 14 days of stopping an MAOI. Your pharmacist can confirm whether a medicine is an MAOI. If you have taken one recently, say so before Qsymia is prescribed. The 14-day gap is not negotiable in the label.
Alcohol and other CNS depressants
The label does not ban alcohol, but it does ask you to keep it modest.
Alcohol, benzodiazepines, barbiturates and sleep medicines
Use cautionThese may add to the dizziness, and to the effects on thinking, that phentermine and topiramate can cause.
CNS depressants are medicines that slow the brain down, and the label names barbiturates, benzodiazepines and sleep medications. It says taking alcohol or a CNS depressant with phentermine or topiramate may deepen that slowing effect. Dizziness is one example, and problems with thinking are another. Its instructions are practical. It says to advise patients not to drive or operate machinery until they know how Qsymia affects them. That judgement covers mental performance, motor performance and vision. It says to caution patients against excessive alcohol intake. And it says to consider a dose reduction, or stopping, if problems with thinking persist. The Medication Guide puts it more plainly: do not drink too much alcohol while taking Qsymia.
Oral contraceptives
The label is specific here, and the specific answer is reassuring.
Combination oral contraceptives
Generally okayThe amount of hormone in the blood shifts, and spotting may be more frequent. But the label does not expect a higher risk of pregnancy.
One study combined Qsymia 15 mg/92 mg with a single dose of an oral contraceptive. That pill contained 35 micrograms of ethinyl estradiol and 1 mg of norethindrone. Exposure to the estrogen fell by 16%, and exposure to the progestin rose by 22%. The label says this interaction is not expected to increase the risk of pregnancy. It does say that irregular bleeding — spotting — may occur more often, because the lining of the womb is held less steady. Its instruction is explicit and worth repeating. Do not stop your combination oral contraceptive if spotting occurs, but tell your healthcare provider if the spotting is troubling you.
Diuretics and potassium
This one comes with a blood test attached.
Non-potassium-sparing diuretics
Use cautionTaken with Qsymia, they may push potassium down further. So potassium is measured before and during treatment.
Some water tablets — the ones called non-potassium-sparing diuretics — flush potassium out of the body. The label says using them with Qsymia may add to that potassium loss. It also reports that hydrochlorothiazide alone with topiramate alone raised topiramate exposure by 27 to 29%. It says to measure potassium before and during Qsymia treatment when the two are used together. That sits alongside a separate label warning. Qsymia can increase the risk of hypokalaemia — low potassium — in its own right, because topiramate blocks an enzyme called carbonic anhydrase.
Antiepileptic drugs
Topiramate is itself an epilepsy medicine, so this section carries real weight.
Phenytoin, carbamazepine and valproic acid
Use cautionSome of them lower topiramate levels a lot. Valproic acid combined with topiramate has been linked to hyperammonaemia and hypothermia.
The label reports figures from people with epilepsy. Phenytoin decreased the topiramate level in the blood by 48%, and carbamazepine by 40%, compared with topiramate given alone. Hyperammonaemia means too much ammonia in the blood, and encephalopathy means disturbed brain function. The label says taking valproic acid and topiramate together has been associated with hyperammonaemia, with and without encephalopathy. The same combination has been associated with hypothermia, a drop in body temperature, with and without hyperammonaemia. It says to consider measuring blood ammonia in anyone who develops hypothermia or encephalopathy.
Other carbonic anhydrase inhibitors
A class you may not realise you are taking — some glaucoma drops and diuretics belong to it.
Carbonic anhydrase inhibitors
Avoid or discussTaken with topiramate, they may worsen metabolic acidosis — too much acid in the blood — and raise the risk of kidney stones.
The label says one more thing about this class. Using topiramate with any other carbonic anhydrase inhibitor may increase the severity of metabolic acidosis. It may also increase the risk of kidney stones forming. The label says the combination should be avoided. If it cannot be avoided, it says to monitor for metabolic acidosis appearing or getting worse. Related to this, the Medication Guide says to drink plenty of fluids while taking Qsymia. That helps decrease the chance of kidney stones. It also says to call your provider about severe side or back pain, or blood in your urine.
Amitriptyline and pioglitazone
Two named medicines with smaller, more specific effects.
What the label says about each
Use cautionAmitriptyline levels can rise sharply in some people. Pioglitazone exposure falls.
On amitriptyline, the label says some patients may have a large increase in the amitriptyline level when topiramate is present. It says any dose adjustment should be made according to how the patient responds. It should not be based on measured levels. On pioglitazone, the label reports a decrease in exposure to pioglitazone and its active breakdown products when it is used with topiramate. It says the clinical relevance is unknown. It suggests considering more frequent blood sugar checks when the two are used together.
Common questions
Written and maintained by one person · Built with agentic AI tools · Every number checked against the cited trial · Not medical advice
- Qsymia (phentermine and topiramate) prescribing information — Drug Interactions (7, Table 5), Warnings and Precautions (5.7, 5.10, 5.11, 5.12), Medication Guide View source ↗
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